A form of triplate antenna comprises a radiating elment including a pair of closely spaced correspondingly apertured ground planes with an interposed printed film circuit, electrically isolated from the ground planes, the film circuit providing excitation elements or probes within the areas of the apertures, to form dipoles, and a feed network for the dipoles. In an array antenna a plurality of such aperture/element configurations are spaced at regular intervals colinearly in the overall triplate structure. This antenna construction lends itself to a cheap yet effective construction for a linear array antenna such as may be utilised for a cellular telephone base station. Such an antenna is disclosed in our copending patent application Ser. No. 91 24291.7.
Another type of layered antenna array comprises a single aperture per radiating element. A still further type comprises a primary aperture with two secondary apertures placed on opposite sides of the primary aperture. The array may extend in a single direction 9a (linear array) or in two directions (a planar array). In order to increase output from the antenna in a primary radiating direction, the antenna may further comprise an unapertured ground plane placed parallel with and spaced from one of the apertured ground planes to form a rear reflector for the antenna. Signals transmitted by the antenna towards the backplane are re-radiated in a forward direction.
A problem with array antennas having such a reflecting backplane is the need to control coupling between apertures and the feed network. The feed network comprises microstrip tracks arranged on a substrate and acts to feed the patch or probe radiating elements. Ideally, the feed network couples only with the repective probes/radiating elements and does not couple with re-radiated signals received from the reflecting backplane. Careful design of the dimensions of the apertures and the elements coupled with the design of the electrical characteristics of the feed network for the elements can give a measure of control of coupling, but for some applications this is not effective.